Here are the links to references in
UK Aeronautical Information Service (AIS). Subscription is required (free), well worth the effort just to view the valuable safety information. Select, Subscribe here; or (when registered), Publications, UK AIC, Pink(P), for the safety material.
The reference I quoted above (UK CAA AIC 11/98) has been revised; see:-
‘Landing performance of large transport aeroplanes’ 14(P91)/2006, also see
‘Risks and factors associated with operations on runways affected by snow, slush or water’ 15(P92)/2006
The widely quoted speed margin at the threshold of Vref+15 probably originates from the old ‘reference’ method of determining the certified landing performance. Although these tests would have been conducted on a wet runway the resulting performance could vary due to the variability in runway mu, surface, etc.
JT your experiences with a test team may have been exciting, but they reflect the ‘arbitrary’ method of determining the gross (unfactored) landing distance, which as the AIC states is only a theoretical value. For the discarded tests, it is more likely that the test pilot assessed that not all of the parameters were met, thus reflecting the difficult (impossible) task of achieving the unfactored distance in daily operations. I reiterate your warning
be very careful using, unfactored QRH data without thinking carefully what you are doing ..
The most valuable thought is ‘should I be making this approach and landing in these conditions’, not can I make the approach and landing.
The factored landing distance of 1.67 accommodates the FAR “land within 60% of LDA”; JAR OPS requires a further 15% factor for wet runways resulting in a 1.92 safety factor. The resulting distances only provide a safety margin. Safety is never absolute, only relative, i.e. studies in Canada suggest that a factor of 2.2 - 2.4 times the unfactored landing distance is required for some contaminated operations in order to achieve the equivalent margin of safety for landing on a dry runway. There are many pitfalls in the area of relative safety i.e. beware of JAR-OPS allowing wet ‘grooved’ runways being defined as ‘dry’, all this does is reduce the safety margin within ‘acceptable’ limits – acceptable to the authority, but (as usual) the final responsibility rests with the operator / captain.
The use (or abuse) of speed additives is a major factor in determining a safe landing. The AIC concludes
“The highest degree of confidence in successfully achieving the scheduled landing distances is obtained by crossing the threshold at the correct height and at the target threshold speed, touching down firmly after a normal flare and applying maximum retardation without delay.”
Headwind / tailwind corrections are in the tables of factored landing distance (or computed data).
Approach speed additives for wind gusts or turbulence, icing, etc, are operator defined adjustments which have to balance the margin of safety achieved when using Vref, with any reduction of safety margin at a higher speed. The final balance is the overall safety of the operation, thus a speed addition can be safer from a gust/windshear perspective, but less safe from a landing/stopping aspect – check recent accidents. How many stalls on the approach vs the number of overruns!
An aircraft certification requires a demonstration of a safe landing from Vref -5 (in good conditions). Thus a normal approach at Vref +5 already has a 10 kt gust margin, any further addition should be seen as a speed target (something to aim for) and not a minimum. Normally speed adjustments are not accumulative except for specific increases in icing conditions. In these cases the AFM may mandate a longer LDR. The certification may also limit the maximum threshold speed, taking into consideration the chance of a nose-wheel first landing or delayed wt-on-wheels switching that could prevent immediate reverser or brake operation.